The massive size of this mountain of slag, phoptgraphed in the early 1900s, is evidence of the decades of heavy production at the Robesonia Furnace, and of the environmental impact of the iron industry. Iron furnaces piled molten slag, consisting of impurities that separated from iron during the smelting process, into huge mounds. The acids from these mounds then ran off into and polluted surface waters, devastating wildlife.